Process for the production of chlorine dioxide

ABSTRACT

A process for the production of chlorine dioxide by reacting in a reaction vessel an alkali metal chlorate, mineral acid and a reducing agent in such proportions that chlorine dioxide is produced in a reaction medium which is maintained at a temperature of from about 50 DEG C to about 100  DEG C and at an acidity within a range of from about 5 N to about 11 N. The reaction medium is subjected to subatmospheric pressure sufficient for evaporating water, a mixture of chlorine dioxide, oxygen and water vapour being withdrawn from an evaporation zone in the reaction vessel, and alkali metal sulfate being precipitated in a crystallization zone in the reaction vessel. As the reducing agent hydrogen peroxide is used.

The present invention relates to a process for the production ofchlorine dioxide from an alkali metal chlorate, a mineral acid and areducing agent. More closely the invention relates to production ofchlorine dioxide with hydrogen peroxide as the reducing agent. Theprocess is carried out in a vessel operated under subatmosphericpressure, water being evaporated and withdrawn together with chlorinedioxide and oxygen, and the alkali metal salt of the mineral acid beingcrystallized in the reaction vessel and withdrawn therefrom.

Chlorine dioxide used in aqueous solution is of considerable commercialinterest, mainly in pulp bleaching, but also in water purification, fatbleaching, removal of phenols from industrial wastes etc. It istherefore desirable to provide processes in which chlorine dioxide canbe efficiently produced.

In existing processes for the production of chlorine dioxide, chlorinegas is often formed as a by-product, due to the use of chloride ions asreducing agent. The basic chemical reaction involved in such processescan be summarized by the formula

    ClO.sub.3 -+Cl-+2H.sup.+ →ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O[1]

The chlorate ions are provided by alkali metal chlorate, preferablysodium chlorate, the chloride ions by alkali metal chloride, preferablysodium chloride, or by hydrogen chloride, and the hydrogen ions areprovided by mineral acids, generally sulfuric acid and/or hydrochloricacid.

At the production of chlorine dioxide with chloride ions as thereduction agent according to the formula [1] half a mole of chlorine isproduced for each mole of chlorine dioxide. This chlorine gas by-producthas previously been used as such in paper mills as bleaching agent inaqueous solution. However, increased environmental demands have resultetin a change-over to pure chlorine dioxide bleaching in industry. Toachieve pure chlorine dioxide bleaching there is an increasing demandfor chlorine dioxide manufacturing processes which don't producechlorine as a by-product.

One known way of reducing the chlorine by-product is to use otherreducing agents than chloride, reducing agents which don't producechlorine at the reduction. In the so called "Solvay" process alkalimetal chlorate is reduced in an acid medium with methanol as thereducing agent and in the "Mathieson" process the chlorate is reducedwith sulphur dioxide in sulphuric acid containing medium. These reducingagents are indirect reducing agents and their reactions are very slow.In U.S. Pat. No. 4,081,520 a more effective "Solvay" process has beenobtained by use of a reduced pressure and a high acid normality in asingle vessel reactor.

The direct reaction between chlorate ions and methanol or sulphurdioxide is very slow, and the direct reducing agent in these cases arechloride ions reacting according to [1]. The chlorine produced is thenreacting with methanol to regenerate chloride ions according to theformula

    CH.sub.3 OH+3Cl.sub.2 +H.sub.2 O→6Cl.sup.- +CO.sub.2 +6H.sup.+[ 2]

or with sulphur dioxide according to the formula:

    Cl.sub.2 +SO.sub.2 +2H.sub.2 O→2HCl+H.sub.2 SO.sub.4[ 3]

It is therefore often necessary to continuously add a small amount ofchloride ions in order to obtain a steady production. Also with methanoland sulphur dioxide as reducing agents a certain amount of chlorineby-product is produced. According to U.S. Pat. No. 4,081,520 operatingwith methanol as reducing agent, the amount of chlorine by-productproduced is decreased with increasing acid normality in the reactionmedium. The reaction rate is also increased with increasing acidstrength. An acid normality above 9 is recommended. The draw back with ahigh acid strenth in the reaction medium is, besides more corrosion inthe equipment, the production of an acid salt in the form ofsesquisulfate (Na₃ H(SO)₂) or bisulfate (NaHSO₄). An acid salt meansloss of acid in the production and costs for neutralization of the salt.

Another draw back with methanol as the reducing agent could be theformation of chlorinated organic compounds, from by-products ofmethanol, in the bleaching train. It is well known that the efficiencyof the added methanol is lowered due to side reactions whereformaldehyde and formic acid are formed. Also some of the methanolleaves the reactor without having participated in the reduction. Thecorresponding ether and ester are probably there as well. It could beexpected that reactions can occur in the bleaching train with thealdehyde, acid, ether and ester resulting in chlorinated organiccompounds.

It is accordingly an object of the present invention to provide achlorine dioxide process of high efficiency and high production ratewherein little or no chlorine is formed as a by-product. It is a furtherobject of the invention to provide a process wherein no otherdetrimental by-products are formed.

The object of the invention was achieved by a process, as claimed in theclaims, for producing chlorine dioxide using a single vessel processwith a reduced pressure and with hydrogen peroxide as the reducingagent. It was surprisingly found that with hydrogen peroxide as thereducing agent it is possible to obtain a process with a very highreaction rate and efficiency. With the process according to theinvention an essential chlorine free process is obtained.

Hydrogen peroxide is known as a reducing agent in the literature. In"Wasserstoffperoxid und seine Derivative Chemie und Anwendung" hydrogenperoxide is reported as a reducing agent for chlorate in chlorinedioxide production. The advantage is said to be the production ofchlorine free chlorine dioxide. However, as a draw back the productionof bisulfate (NaHSO₄) is mentioned. This means that it has been regardedas necessary to have an acid normality above about 11N in the reactionmedium. The following formula is given:

    2NaClO.sub.3 +2H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 →2ClO.sub.2 +2NaHSO.sub.4 +2H.sub.2 O+O.sub.2

Bisulfate is the salt obtained when the acid normality is above 11N.

In the Japanese patent application JP 88-8203 chlorine dioxideproduction is obtained with an acid strength of 8 to 11N with hydrogenperoxide and chloride as reducing agents. The necessary amount ofchloride is given to 0.02 to 0.1 mol/l.

In the present invention it was found that if the single vessel processwith reduced pressure was used together with hydrogen peroxide it waspossible to produce essentially chlorine free chlorine dioxide at acidnormalities from about 5 to about 11N and thus produce a less acidicsalt than bisulfate. The following formula illustrate the reaction:

    6NaClO.sub.3 +4H.sub.2 SO.sub.4 +3H.sub.2 O.sub.2 →6ClO.sub.2 +2Na.sub.3 H(SO.sub.4).sub.2 +2H.sub.2 O+O.sub.2

It was found that the reaction rate and the efficiency was very high inthe whole range 5 to 11N.

As said above according to prior art it is conventional to add a smallamount of chloride ions to obtain a steady production. Usually theaddition is in the range of 0.5 to 5% by weight of sodium chloride tothe sodium chlorate before it is used in a single vessel process withreduced pressure and methanol as the reducing agent. In Japanese patentapplication JP 88-8203 it is necessary to have 0.02 to 0.1 mol/l ofchloride together with hydrogen peroxide. However, with the presentprocess it was possible to produce chlorine dioxide without anysubstantial addition of chloride ions, thereby obtaining an essentiallychlorine free process.

Another advantage with hydrogen peroxide as reducing agent for chlorinedioxide production is the by-product of oxygen. Besides the by-productbeing a harmless gas, this oxygen can be used in the paper mills as ableaching agent together with the chlorine dioxide. It is anotherpreferred embodyment of the present invention to use the chlorinedioxide produced according to the present invention together with all ora part of the formed oxygen in the paper mills as bleaching agents.

The amount of hydrogen peroxide for the reduction of alkali metalchlorate can be from about 0.06 to about 0.6, suitably about 0.16 toabout 0.32 ton/ton chlorate, preferably 0.16 to 0.22 ton/ton chlorate.

The chlorate concentration in the reaction vessel can vary within widelimits, from a low concentration of about 0.25M up to saturation,preferably from about 0.4M up to saturation, most preferably from about0.7M to about 2.5M.

The present process is an essentially chlorine free process. Nosubstantial amount of chloride ions are added. The chlorate used in theprocess is conventional, commercially available, chlorate. By way ofmanufacturing such chlorate always contains a small amount of chloride.That amount of chloride is not more than about 0.5, often not more thanabout 0.05, preferably not more than about 0.02, most preferably notmore than about 0.01 weight percent alkali metal chloride. Beside thisamount of chloride being an impurity in the chlorate no further chlorideis added. There is also commercially available chlorate with higheramounts of chloride. That chlorate has been obtained by adding extraalkali metal chloride to the chlorate. Such a chlorate is not suitablefor the present process.

The production of chlorine dioxide according to the present process isperformed in a single reaction vessel,generator-evaporator-crystallizer. A suitable reactor is an SVP.sup.(R)(single vessel process) reactor. The reactants are added continuously tothe reactor. The alkali metal chlorate is added in an amount of from1.58 to 2.0 tone per tone chlorine dioxide and hydrogen peroxide in anamount within the interval as mentioned above. The reaction is suitablyoperated at a temperature of 50°-100° C., preferably 50°-75° C. and at apressure below atmospheric pressure, suitably at 60-400 mm Hg. Then thereaction medium boils or water is evaporated in a sufficient amount todilute the chlorine dioxide formed to a safe concentration. The acidityin the reactor is adjusted by adding a mineral acid, preferably sulfuricacid. In the reactor, the alkali metal salt of the mineral acid iscontinuously crystallized and separated in a suitable manner. Theprocess is not restricted to any of the alkali metals, but sodium is themost preferred.

The acidity of the reaction medium can be within a the whole range5-11N. However, it is preferred to run the reaction at an acidity belowabout 9N.

If suitable it is also possible to add other reducing agents such asmethanol, formaldehyde, formic acid, sugar alcohols, sulfur dioxide andchloride. Catalyst such as silver, manganese, vanadium, molybdenum,palladium and platinum can also be added if appropriate.

The invention is illustrated by means of the following examples, whereparts and percentages mean parts by weight and percentages by weight,unless otherwise specified.

EXAMPLE 1

To a laboratory chlorine dioxide generator a water solution of 379 g/hNaClO₃ was continuously added together with 207 g/h H₂ O₂ of 30%concentration. H₂ SO₄ of 50% concentration was fed in an amountsufficient to keep an acid strength of 8.3N. 0.15 g/h NaCl was alsoadded together with the chlorate solution (emanating from chlorideimpurity of the conventional chlorate). The generator was runcontinuously at a temperature of about 70° C. and the reaction mediumwas kept at boiling when the absolute pressure was 150 mm Hg, i.e. belowatmospheric pressure. The chlorine dioxide production was 1.9 kg/l and24 hr and the yield was 100%.

We claim:
 1. In a process of producing chlorine dioxide by reducingalkali metal chlorate with hydrogen peroxide as the reducing agent in anaqueous reaction medium containing sulfuric acid, said reaction mediummaintained in a single reaction vessel under subatmospheric pressure,the process including the steps of maintaining said reaction medium atits boiling point at a temperature of from about 50° C. to about 100° C.to evaporate water from said reaction medium and provide a gaseousmixture containing steam, oxygen and chlorine dioxide;removing saidgaseous mixture from said single reaction vessel, recovering an aqueoussolution of chlorine dioxide from said removed mixture; maintainingsubstantially steady state conditions in said reaction medium bycontinuously feeding said alkali metal chlorate, hydrogen peroxide andsulfuric acid into said reaction medium to make up chemicals consumed insaid reducing step and to maintain a total acid normality in saidreaction medium within the range of from about 5N to about 11N;maintaining the liquid level in said single reaction vesselsubstantially constant by balancing water fed to the single reactionvessel with water removed therefrom; continuously depositing alkalimetal sulfate salt from said reaction medium after the reaction mediumbecomes saturated thereby after the initial start up of the process, andremoving said deposited alkali metal salt from said single reactionvessel; wherein said process is carried out in the substantial absenceof added chloride ions.
 2. A process according to claim 1, whereinhydrogen peroxide is used in an amount of about 0.06 to about 0.6ton/ton chlorate.
 3. A process according to claim 1, wherein hydrogenperoxide is used in an amount of about 0.16 to about 0.32 ton/tonchlorate.
 4. A process according to claim 1, wherein the chlorateconcentration is from about 0.25M up to saturation in the reactionvessel.
 5. A process according to claim 1, wherein the chlorateconcentration is from about 0.4M up to saturation in the reactionvessel.
 6. A process according to claim 1, wherein the chlorateconcentration is from about 0.7M to about 2.5M in the reaction vessel.7. A process according to claim 1, wherein conventional alkali metalchlorate without extra added alkali metal chloride is used.
 8. A processaccording to claim 1, wherein the alkali metal is sodium.
 9. A methodfor bleaching paper pulp using a blend of chlorine dioxide and oxygen,comprising the steps of:(a) forming a gaseous mixture of chlorinedioxide, oxygen and steam by reducing alkali metal chlorate withhydrogen peroxide as the reducing agent in an aqueous medium containingsulfuric acid, said reaction medium maintained in a single reactionvessel under subatmospheric pressure and at an acid normality of fromabout 5N to about 11N, wherein the gaseous mixture of chlorine dioxide,oxygen and steam is formed by the steps of (1) maintaining the reactionmedium at its boiling point at a temperature of from about 50° C. toabout 100° C. to evaporate water from the reaction medium and to therebyform the gaseous mixture; (2) removing the gaseous mixture from thesingle reaction vessel and recovering oxygen and an aqueous solution ofchlorine dioxide from said removed mixture; (3) continuously depositingalkali metal sulfate salt from the reaction medium after the reactionmedium becomes saturated thereby, and removing the deposited alkalimetal salt from the single reaction vessel: wherein said formation ofchlorine dioxide is carried out in the substantial absence of addedchloride ions, and (b) bleaching the paper pulp with the chlorinedioxide solution and with at least part of the oxygen formed during thereduction of alkali metal chlorate in step (a).